Coccidiosis is a disease which is caused by intracellular parasites, protozoa, of the subphylum Apicomplexa and the genus Eimeria. These parasites multiply in cells which form part of the gastrointestinal tract and digestive organs.
Due to the increase in intensive production, the damage which is caused by these parasites in the poultry industry has risen alarmingly in recent decades. For example, the losses which poultry farmers in the Netherlands suffer every year run into millions of guilders; the loss in 1986 was about 13 million guilders; in the same year a loss of U.S. $300 million was suffered in the U.S., despite the use of coccidiostats.
The pathogens of coccidiosis in chickens can be subdivided into nine different species, i.e. Eimeria acervulina, E. maxima, E. tenella, E. necatrix, E. brunetti, E. mitis, E. praecox, E. mivati and E. hagani. However, some people doubt the existence of the last two species. All of these species have only the chicken as host and display a high degree of tissue specificity. The life cycles of the said species are, however, similar.
The species do differ in their pathogenic effect on chickens, the type of chicken also playing a role; thus, a broiler chicken will be subjected to a great deal of damage by a parasite such as E. acervulina or E. maxima because these parasitise large portions of the small intestine, where food digestion plays a major role.
During the life cycle, the Eimeria parasites pass through a number of stages. The infectious stage (the sporulating oocyst) is taken in orally and passes into the stomach of the chicken, where the wall of the cyst bursts open as a result of the grinding action. The four sporocysts, which this oocyst contains, are released and pass into the duodenum, where they are exposed to bile and digestive enzymes. As a result, an opening is made in the sporocyst wall and the sporozoites present in the sporocyst are released. These sporozoites are mobile and search for suitable host cells, epithelium cells, in order to penetrate and to reproduce. Depending on the species, this first reproduction phase lasts 20 to 48 hours and several tens to hundreds of merozoites are formed, which each again penetrate a new host cell and reproduce. After two to sometimes five of these asexual reproduction cycles, depending on the species the intracellular merozoites grow into sexual forms, the male and female gametocytes. After fertilization of the female by a male gamete, a zygote is formed which creates a cyst wall about itself. This oocyst leaves the host cell and is driven out with the faeces. If the temperature and humidity outside the chicken are relatively high and, at the same time, there is sufficient oxygen in the air, the oocyst can sporulate to the infectious stage.
Thus, no intermediate host is needed for transfer of the parasite from chicken to chicken. It is therefore conceivable that with a high degree of occupation of the available surface area the infection pressure in a chicken farm rapidly increases.
The parasite can be combatted in various ways.
In addition to using good management, coccidiosis can be controlled by using coccidiostatic agents which frequently are mixed in the feed or drinking water. However, these agents have suffered a drop in effectiveness in recent years, partly because of the high genetic capacity of the parasite to develop resistance against various combatting agents. In addition, a number of these agents leave residues in the meat which can give rise to problems on consumption.
Immunological prophylaxis would, therefore, constitute a much better combatting method. It is known that chickens which have lived through a sufficiently high infection are able to resist a subsequent contact with the same type of Eimeria. Resistance towards Eimeria can also be induced by infecting the birds several times with low doses of oocysts or with oocysts of weakened (non-pathogenic) strains. However, controlled administration to, specifically, large numbers of broiler chickens is a virtually insurmountable problem in this case.